A method and a device are discussed in European Patent Application EP 0 783 101 A2 for the optical concentration measurement of fine dust in a medium. In this instance, polarized light is specifically supplied to the medium containing the particles, and then the scattered light intensities of the scattered light that is coupled out are measured. This known device for the optical measurement of the concentration of fine dust has the disadvantage that several polarization filters and collimation lenses are required, and therefore the manufacture of such a device is connected with a comparatively great expenditure.